Octane number control of distillation column bottoms by varying heat input

ABSTRACT

A FRACTIONAL DISTILL ATION COLUMN OPERATION AS A GASOLINE STABILIZER OR SPLITTER IS CONTROLLED BY MEASURING THE OCTANE NUMBER OF THE COLUMN BOTTOMS AND ADJUSTING THE HEAT INPUT TO THE COLUMN IN RESPONSE TI THE OCTANE NUMBER. THE OCTANE MEASUREMENT IS EFFECTED BY THE ANALYZER COMPRISING A STABILIZED COOL FLAME GENERATOR WITH SERVO-POSITIONED FLAME FRONT WHICH PROVIDES A REAL TIME OUTPUT SIGNAL INDICATIVE OF SAMPLE OCTANE NUMBER.

March ,7, 1972 w BAJEK ETAL OCTANE NUMBER CONTROL OF DISTILLATION COLUMN BOTTOMS BY VARYING HEAT INPUT Filed Oct. 22, 1969 N what $98. 0 ma mmum m M kw H 1hr- Flu: xoxbxmtww mwk 9m B@\ 53.6% mSEuQ Q I 3 5 I m mm mm A TTORNEYS @H mm 1,

Walter A. Bajak By: James H. McLaug/r/m Ee mw United States Patent Oflice 3,647,634 Patented Mar. 7, 1972 3,647,634 OCTANE NUMBER CGNTROL F DISTILLATION COLUMN BOTTOMS BY VARYING HEAT INPUT Walter A. Bajek, Lombard, and James H. McLaughlin, La Grange, Ill., assignors to Universal Oil Products Company, Des Plaines, Ill.

Filed Oct. 22, 1969, Ser. No. 868,458 Int. Cl. B0141 3/42 US. Cl. 196-132 7 Claims ABSTRACT OF THE DISCLOSURE A fractional distillation column operating as a gasoline stabilizer or splitter is controlled by measuring the octane number of the column bottoms and adjusting the heat input to the column in response to the octane number. The octane measurement is effected by the analyzer comprising a stabilized cool flame generator with servo-positioned flame front which provides a real time output signal indicative of sample octane numlber.

BACKGROUND OF THE INVENTION The invention of this application is a process control application of the hydrocarbon analyzer described in United States Patent No. 3,463,613, all the teachings of which, both general and specific, are incorporated by reference herein.

As set forth in application Ser. No. 471,670, the composition of a hydrocarbon sample can be determined by burning the sample in a combustion tube under conditions to generate therein a stabilized cool flame. The position of the flame front is automaticaly detected and used to develop a control signal which, in turn, is used to vary a combustion parameter, such as combustion pressure, induction zone temperature or air flow, in a manner to immobilize the flame front regardless of changes in composition of the sample. The change in such combustion parameter required to immobilize the flame following a change of sample composition is correlatable with such composition change. An appropriate read-out device connecting therewith may be calibrated in terms of the desired identifying characteristic of the hydrocarbon sample, as, for example, octane number. Such an instrument is conveniently identified as a hydrocarbon analyzer comprising a stabilized cool flame generator with a servopositioned flame front. The type of analysis effected thereby is not a compound-by-compound analysis of the type presented by instruments such as mass spectrometers or vapor phase chromatographs. On the contrary, the analysis is represented by a continuous output signal which is responsive to and indicative of hydrocarbon composition and, more specifically, is empirically correlatable with one or more conventional identifications or specifications of petroleum products such as Reid vapor pressure, ASTM or Engler distillations or, for motor fuels, knock characteristics such as research octane number, motor octane number or composite of such octane numbers.

For the purpose of the present application, the hydrocarbon analyzer is further limited to that specific embodiment which is designed to receive a hydrocarbon sample mixture containing predominantly gasoline boiling range components, and the output signal of which analyzer provides a direct measure of octane number, i.e. research octane, motor octane or a predetermined composite of the two octane ratings. For brevity, the hydrocarbon analyzer will be referred to in the following description and accompanying drawings simply as an octane monitor.

An octane monitor based on a stabilized cool flame generator possesses numerous advantages over conventional octane number instruments such as the CFR engine or automated knock-engine monitoring systems. Among these are: examination of moving ports with corresponding minimal maintenance and down-time; high accuracy and reproducibility; rapid speed of response providing a continuous, real-time output; compatibility of output signal with computer or controller inputs; ability to receive and rate gasoline samples of high vapor pressure, e.g. up to as high as 500 p.s.i.g., as well as lower vapor pressure samples (5-250 p.s.i.g.). These characteristics make the octane monitor eminently suitable not only for an indieating or recording function, but particularly for a process control function wherein the octane monitor is the primary sensing element of a closed loop control system comprising 0, 1, 2 or more subloops connected in cascade.

The present invention has as its principal objective the direct control of octane number of a stabilizer or splitter column bottoms stream. A stabilizer is an externally refluxed, multiple tray, fractional distillation column employed to remove the light ends from unstabilized or :wild gasoline. For example, the feed to such a column is, typically, unstabilized reformate from a catalytic naphtha hydroreforming unit. The hot vaporous reactor effluent containing hydrogen, normally gaseous hydrocarbons and gasoline boiling range hydrocarbons is condensed and passed to a separation zone which is a single stage gravity-type phase separator maintained at reforming pressure range of, say 50-500 p.s.i.g. The liquid hydrocarbon or unstabilized reform ate phase is in equilibrium therein with the gas phase containing a major proportion of hydrogen. The hydrogen is withdrawn and a portion thereof is recycled to the inlet of the catalytic reforming zone together with the naphtha charge. The liquid hydrocarbon phase from the separator is the feed to the stabilizer column. It contains a minor proportion of dissolved hydrogen and C -C hydrocarbons which must be removed in order that the stabilized reformate will meet vapor pressure and octane number specifications. A typical sample of catalytic reformate from a separator operat- The overhead from the column is predominantly C and lighter hydrocarbons and the column bottoms is stabilized gasoline comprising predominantly C to about 400 F. endpoint material. By and large it has been the practice to operate such a column mostly in the dark" so far as product octane number is concerned. That is to say, the column bottoms is manually sampled perhaps once every eight hour shift or perhaps even only once a day. The sample is picked up and taken to the laboratory where the sample is run and the result then transmitted back to the unit operator who, until then, has not been able to ascertain What change, if any, should have been made at the time the sample was taken. Therefore, to be on the safe side, the unit operator will usually run the column with excessive heat input and with corresponding over-reflux whereby the stabilized reformate will actually be outside of product specifications with respect to octane number a good part of the time. This mode of operation clearly increases the refiners costs.

The control problem is further complicated by the not uncommon practice of using a single stabilizer to process more than one gasoline stream. For example, a single stabilizer column will often receive plural or combined feeds which are unstabilized reformates from two or more independently operated catalytic naphtha reforming units. An upset in the operation of a single such reformer will carry through to the stabilizer and be reflected in off-specification product so that the stabilizer bottoms product is no longer indicative of only the operation of a single reformer. Continuously meeting octane number specification is an exceedingly diflicult and haphazard task when employing a single stabilizer column to handle a plurality of gasoline streams.

SUMMARY OF THE INVENTION It is therefore an object of the present invention to provide an improved control system for use and in combination with a continuous flow reboiled fractional distillation column.

It is another object of the present invention to provide such an improved control system for a fractional distillation column operating on one or more unstabilized gasoline feed stocks.

It is a further object of the present invention to provide an improved control system for maintaining such a fractional distillation column under operating conditions sufiicient to produce a bottoms fraction comprising a stabilized gasoline product having a substantially constant predetermined octane number.

These and other objectives of the present invention, as well as the advantages thereof, will be more clearly understood as the invention is more particularly disclosed hereinafter.

In accordance with the present invention, the octane monitor comprising a stabilized cool flame generator with servo-p'ositioned flame front is connected to receive a continuous sample of the column bottoms. The output signal of the octane monitor, which can be, and preferably is, calibrated directly in terms of octane number, is utilized to reset or adjust reboiler heat input to the column so that the bottoms octane number is maintained at a substantially constant predetermined level. This control system assures that the stabilized reformate is always on specification, regardless of upsets or disturbances, and further effects a savings in utility costs in that the stabilizer is then operated at minimum heat input and minimum reflux.

Because there is a direct measurement and control of octane rating, this control system is to be distinguished from those prior art control system wherein some composition property, such as percent aromatics or conductivity or dielectric constant, is measured and controlled, all of these latter properties being merely an indirect indication of octane rating which is only narrowly correlatable therewith. Such indirect correlation becomes invalid for any significant deviation from the design control point.

The control system of this invention is also to be distinguished from those prior art systems employing automated knock-engines as the octane measuring device. The instant octane monitor is compact in size, can be totally enclosed by an explosion-proof housing and therefore can be used in hazardous locations. In fact it is normally field-installed immediately adjacent the stabilizer column. A knock-engine, however, cannot be employed in hazardous locations and must therefore be situated remote from the sample point. The sample transport lag or dead time of a close-coupled octane monitor is typically of the order of two minutes, and its 90% response time is another two minutes. This is a very good approach to an essentially instantaneous or real time output.

By way of contrast, the transport lag alone of a knockengine may be of the order of thirty minutes or more, which those skilled in the control system art will recognize to be a substantial departure from real time out- 4 put. With that much dead time built into a closed loop, it is extremely difficult to achieve and maintain stability. The injection of an outside disturbance of any appreciable magnitude, in such a potentially unstable system, will often result in undampened cycling with the consequence that the system will have -to be put on manual control.

In a broad embodiment, the present invention is directed to a control system for use and in combination with a continuous flow fractional distillation column, the feed to which is a gasoline fraction, the overhead from which comprises the lower boiling components of said fraction and the bottoms from which comprises the higher boiling components of said fraction, said column including a reboiler section having a vapor outlet connecting with the lower portion of the column and means to supply heat to said reboiler from an extraneous heat source, said system comprising (1) means operatively associated with said heat supply means to vary the heat input to said reboiler; (2) a hydrocarbon analyzer comprising a stabilized cool flame generator with a servo-positioned flame front continuously receiving a sample of said column bottoms and developing an output signal which in turn provides a measure of sample octane number; and, (3) means transmitting said analyzer output signal to said heat input varying means whereby the heat input to said column is regulated responsive to octane number of said column bottoms and said octane number is thereby maintained at a substantially constant predetermined level.

Preferred specific embodiments will incorporate one or more cascaded subloops which more immediately control the heat input to the column. For example, where the stabilizer reboiler is indirectly heated by a fluid heating medium such as steam, reactor effluent or hot oil, there may be a flow control loop on the heating medium to the reboiler, the octane monitor output then being cascaded to the flow controller setpoint. Alternately, reboiler vapor temperature control may reset the flow controller and the octane monitor outputv will reset such temperature controller setpoint. Other embodiments will be apparent in light of the detailed description of the invention.

The invention will be more clearly understood by reference to the accompanying drawings in which:

FIG. 1 illustrates a stabilizer column together with one mode of controlling the heat input thereto; and,

FIG. 2 is a fragmentary view of a stabilizer column with a triple cascade system for regulating heat input to maintain constant bottoms octane number.

DESCRLPTION OF THE FIGURES With reference now to FIG. 1, there is shown a stabilizer column 4 receiving a plurality of unstabilized gasoline feeds. Stabilizer column 4 is a conventional continuous flow externally refluxed fractional distillation column containing from 10 to 50 or more vertically spaced vaporliquid contacting stages as, for example, bubble decks, sieve decks, perforated trays or the like. Line 1 carries unstabilized reformate from naphtha reforming unit No. 1. Line 2 carries unstabilized reformate from naphtha reforming unit No. 2. The combined reformates are charged to the column via line 3 which connects with the column at a locus approximately midway in the height thereof.

The two reforming units are separate, independently operated catalytic naphtha reforming units; the details thereof form no part of the present invention, being conventional and well known in the art. A typical catalytic naphtha hydroreforming unit is described in U.S. Patent 3,296,118 (Class 208l00) to which reference may be had for specific information concerning flow arrangement, catalyst, conditions etc. The feed to column 4 is generally under reactor products separator level control rather than direct flow control. Accordingly, the feed rate is usually but not always relatively constant, and may be subject to some variation due to changes in catalyst and/ or operating conditions in either or both of the catalytic reforming units.

The overhead material from column 4, typically comprising predominantly C and lighter hydrocarbons, together with some hydrogen, is removed via line 5, condensed in overhead condenser 6, and passed via line 7 to overhead receiver 8. Non-condensibles are removed through line 9. Overhead condensate, comprising essentially butanes with some propane, is taken off from receiver 8, a portion thereof being returned through line 10 as reflux to column 4 and the remainder being sent via line 11 to storage. Conventional instrumentation, not here illustrated, will be provided to control column pressure, overhead receiver liquid level, and reflux rate.

Stabilized gasoline product typically comprising C to 400 F. endpoint material is taken off through bottoms drawolf line 12 and sent to storage or to further processing. (As used herein, the term end point and the temperatures illustrated are those typically defined by lavoratory distillation in accordance with ASTM Method D- 86 A portion of stabilizer bottoms is recirculated via line 13 through reboiler 14 and then through vapor transfer line 15 back to the lower portion of column 4. Reboiler 14 may be a shell and tube heat exchanger, as illustarted, or a stab-in internal reboiler or a fired heater. Bottoms circulation may be forced convection or thermosyphon. A suitable fluid heating medium, for example steam, is passed through the tube side of reboiler 14 via lines 16 and 17. The steam flow rate is regulated by a flow control loop comprising orifice 19, rflow signal line 20, flow controller 18, controller output line 21 and valve 22. The setpoint of flow controller 18 is automatically adjustable.

A temperature controller 23, also with automatically adjustable setpoint, senses and controls reboiler vapor temperature as detected by a sensing means such as thermocouple 25 located in vapor outlet line 15. The resulting temperature output signal is transmitted via line 24 to adjust or reset the setpoint of flow controller 18.

Octane monitor 26, utilizing a stabilized cool flame generator with servo-positioned flame front, is field-installed adjacent to column 4. In a preferred embodiment, the flows of oxidizer (air) and fuel (gasoline sample) are fixed as is the induction zone temperature. Combustion pressure is the parameter which is varied in a manner to immobilize the stabilized cool flame front. Upon a change in sample octane number, the change in pressure required to immobilize the flame front provides a direct indication of the change in octane number. Typical operating conditions for the octane monitor are:

Air flow3500 c./ min. (STP) Induction zone temperature-700 F. (research octane),

800 F. (motor octane) Combustion pressure-440 p.s.i.g.

Octane range (max.)80l02 1 1 The actual calibrated span of the octane monitor as here utilized will, in general, be considerably narrower. For Example. if the target octane is 95 clear (research method), a suitable span may be 92-98 research octane. When a relatively narrow span is employed, the octane number change is essentially directly proportional to the change in combustion pressure.

Dashed line 28 represents a suitable sampling system to provide a continuous sample of column bottoms to the octane monitor. For example, the sampling system may comprise a sample loop taking bottoms at a rate of 100 cc. per minute from a point upstream of a control valve and returning it to a point downstream from the control valve, the sample itself being drawn off from an intermediate portion of the sample loop and injected at a controlled rate by a metering pump to the combustion tube of the octane monitor.

The octane monitor output signal is transmitted via line 27 to the setpoint of temperature controller 23. This may be a direct field connection, but preferably the octane monitor output will first be sent to an octane controllerrecorder located in the refinery control house, and the control signal therefrom then being sent to reset the setpoint of temperature controller 23 which may be a temperature recording controller also located in the control house.

Upon a decrease in the measured octane number of the bottoms product, the octane monitor will call for an increase in reboiler vapor temperature to drive out a greater proportion of the light ends from the column bottoms. Temperature controller 23 will then call for an increase in steam flow which in turn will be elfected by flow controller 18. If the octane number deviation is in the opposite direction, e.g., becomes higher than octane specification, the octane monitor will call for a decrease in reboiler vapor temperature, and the overall corrective action will be the reverse of that previously described. In either event, the octane number of the stabilized gasoline is continuously maintained at a substantially constant predetermined level.

FIG. 1 further illustrates an alternate embodiment wherein the temperature controller 23 senses and controls not the reboiler vapor as it emerges directly from reboiler 14, but rather the liquid or vapor temperature obtaining within the column at a point some distance above the reboiler vapor line and below the feed inlet. In this instance. a thermocouple 25' is used in lieu of thermocouple 25, the former being located several trays (for example, 2-6 trays) above vapor outlet 15. This latter arrangement will afford a more immediate detection of increasing light ends concentration, resulting for example from an over-reflux condition, at least several minutes before such light ends reach the reboiler to cause a change in the operation thereof.

While the double cascade arrangement illustrated in FIG. 1 represents a preferred embodiment, it is within the scope of this invention to omit the temperature controller 23 and to reset flow controller 18 directly by the octane monitor output signal transmitted via line 27. Alternatively, the flow controller 18 could also be omitted, in which case octane monitor output signal line 27 would connect directly with valve 22. It may be expected, however, that elimination of either or both of the subloops will result in somewhat poor overall control because reboiler temperature and steam flow variations will become a source of additional upsets and also because the relatively large time constant of the stabilizer column itself tends to make single loop control unstable.

FIG. 2 illustrates still another embodiment of the invention which differs in the provision of a second temperature controller. Here a thermocouple 30, located several trays above vapor outlet line 15, provides a temperature input signal to the temperature controller 29 which has an adjustable setpoint. The temperature output signal from temperature control 29 is transmitted via line 31 to reset the setpoint of temperature controller 23. Controller 23 obtains its input signal from thermocouple 25 located in transfer line 15. The octane monitor output signal is transmitted via line 27 to reset the adjustable setpoint of temperature controller 29. The construction and operation of the remaining components of the control system are the same as previously described in connection with FIG. 1. The triple cascade system of FIG. 2 will be particularly advantageous when it is anticipated that the feed rate and/or feed composition to the stabilizer column will be subject to substantial variation.

PREFERRED EMBODIMENTS Those skilled in the art will readily perceive the method of operation for the inventive control system, as well as the advantages to be accrued therefrom, by the disclosure set forth hereinabove.

Those skilled in the art will furthermore perceive that the inventive control system is not limited to the specific embodiments disclosed For example, the stabilizer column 4 could be operated as a deethanizer, a depropanizer, a depentanizer, or a dehexanizer and still derive the operational benefits of the inventive control system. Furthermore, column 4 could be operated as a gasoline splitter column, wherein a gasoline feed fraction would be separated into low boiling and high boiling constitucuts for subsequent gasoline blending purposes as, for example, wherein a debutanized gasoline is split into an overhead fraction having an endpoint of 380 F. and a bottoms fraction comprising hydrocarbons boiling above 380 F. Additionally, the application of the inventive control system is not limited to the fractional distillation of. reformate gasolines, much less to those having a 400 F. endpoint, and the feed to column 4 could comprise cracked gasoline, natural gasoline, alkylate gasoline, etc. wherein the gasoline feed could be stabilized or unstabilized.

These and other modifications in the fractionation environment should in no way be construed to impose a limitation on the broadness of application of the inventive control system.

It may be summarized, however, that a preferred embodiment of the present invention is a control system for use and in combination with a continuous flow, refluxed fractional distillation column, the feed to which is unstabilized gasoline, the overhead from which comprises predominantly C and lighter hydrocarbons and the bottoms from which is stabilized gasoline, said column including a reboiler section having a vapor outlet connecting with the lower portion of the column and means to supply heat to said reboiler from an extraneous heat source, wherein the control system for said column comprises: (1) means operatively associated with said heat supply means to vary the heat input to said reboiler; (2) a hydrocarbon analyzer comprising a stabilized cool flame generator with a servo-positioned flame front continuously receiving a sample of said column bottoms and developing an output signal which in turn provides a measure of sample octane number; and, (3) means transmitting said analyzer output signal to said heat input varying means whereby the heat input to said column is regulated responsive to Octane number of said column bottoms and said octane number is thereby maintained at a substantially constant predetermined level.

From the foregoing disclosure, it will be obvious to those skilled in the art that the inventive control system provides stable operation of a fractional distillation column in a manner sufiicient to consistently produce a gasoline product of constant predetermined octane number in a more facile, efficient, and economical manner.

We claim:

1. In combination with a continuous flow fractional distillation column, the feed to which is a gasoline fraction, the overhead from which comprises the lower boiling components of said fraction and the bottoms from which comprises the higher boiling components of said fraction, said column including a reboiler section having a vapor outlet connecting with the lower portion of the column and meansto supply heat to said reboiler from an extraneous heat source, a control system for said column comprising:

(1) means operatively associated with said heat supply means to vary the heat input to said reboiler;

(2) a hydrocarbon analyzer comprising a stabilized cool flame generator with a servo-positioned flame front continuously receiving a sample of said column bottoms and developing an output signal which'in turn provides a measure of sample octane number; and,

(3) means transmitting said analyzer output signal to said heat input varying means whereby the heat input to said column is regulated responsive to octane number of said column bottoms and said octane number is thereby maintained at a substantially constant predetermined level.

2. The system of claim 1 wherein the feed to said column comprises unstabilized gasoline, the overhead comprises C and lighter hydrocarbons, and the bottoms cornprises stabilized gasoline containing C and heavier hydrocarbons.

3. The system of claim 1 wherein said heat input varying means comprises a flow control loop including a flow controller having an adjustable setpoint regulating the rate of flow of heating medium through said reboiler, said setpoint being adjusted in response to said analyzer output signal.

4. The system of claim 3 further characterized in the provision of means to sense the temperature in said column at a locus below the feed inlet thereto, temperature control means having an adjustable setpoint connecting with said temperature sensing means and developing a temperature output signal, and means transmitting the last-mentioned output signal to the set point of said flow controller, said means (3) transmitting said analyzer output signal to the temperature controller setpoint whereby the latter is adjusted responsive to bottoms octane number.

5. The system of claim 4 wherein said temperature sensing means is located in the vapor outlet from said reboiler.

6. The system of claim 4 wherein said temperature sensing means is located several trays above said vapor outlet.

7. The system of claim 3 further characterized in the provision of a first means to sense the temperature in said column at a locus below the feed inlet thereto and several trays above said reboiler vapor outlet, a second means to sense the temperature in said reboiler vapor outlet, a first temperature control means with adjustable setpoint connecting with said first sensing means and developing a first temperature output signal, a second temperature control means with adjustable setpoint connecting with said second sensing means and developing a second temperature output signal, means transmitting said first output signal to the setpoint of said second temperature controller, means transmitting said second output signal to the set point of said flow controller, said means (3) transmitting said analyzer output signal to the first temperature controller setpoint whereby the latter is adjusted responsive to bottoms octane number.

References Cited UNITED STATES PATENTS 3,269,922 8/1966 Price et al 203-3 3,336,205 8/ 1967 Rijnsdorp et al 2033 X 3,361,646 1/1968 MacMullan et al. 196-132X 3,401,092 9/1968 Matta 203-1 3,429,805 2/ 1969 Karbosky 208351 X 3,463,613 8/1969 Fenske et al. 23-253 NORMAN YUDKOFF, Primary Examiner D. EDWARDS, Assistant Examiner U.S. C1. X.R.

202l60; 2033, DIG. 18; 208--DIG. l, 95, 351 

